Recently, television receivers are also used as output display sections of microcomputer apparatuses. However, the vertical frequency of an output video signal which is outputted from the microcomputer apparatus is determined from the number of characters in one line or the like and is not limited to the standard frequency of 60 Hz but differs within a range of, for example, 50 to 60 Hz.
In the case of supplying such a video signal to the television receiver, there occur the problems such as inferior vertical linearity, inferior horizontal pin distortion, vertical pull-out, overamplitude of or lack of amplitude of vertical deflection, and the like.
For example, assuming that the frequency of a vertical sync signal V.sub.sy shown in FIG. 5A increased as shown by broken lines than that represented by a solid line, the amplitude of the vertical deflection signal decreases as shown by a broken line in FIG. 5B. Thus, the vertical deflection amplitude lacks.
There has conventionally been known the method whereby the change in vertical amplitude due to the difference in vertical frequency between the NTSC and CCIR systems is corrected by switching the value of the circuit element by a manual switch. However, according to this constitution, it is necessary to operate the manual switch and the vertical amplitude cannot be automatically corrected, so that it is improper for correction of the vertical amplitude when the output video signal of the microcomputer apparatus is displayed.
To automatically correct the vertical amplitude, it is necessary to detect the variation in vertical amplitude corresponding to the vertical frequency of the input signal. As an example of detection of the change of the vertical frequency, there has been known the method whereby a monostable multivibrator is triggered by a vertical sync signal and its output signal is integrated and the vertical frequency is determined from the level of the integrated output. However, since the vertical frequency is fairly low, the value of the capacitor for integration is large and there is a drawback such that response speed is low when the vertical frequency changes.